When a nonlinear optical crystal or any optical element is irradiated with an intensive laser beam, the performance of the crystal or optical element may degrade with time. The higher the incident laser beam intensity, the faster it degrades. This degradation process is usually referred to as laser induced damage. Laser induced damage may happen on the surface of or inside an optical element.
There are many mechanisms leading to laser induced damage in optical element. Some mechanisms lead to instance damages, while some other mechanisms lead to degradation over time in the early stage before a complete damage. For high intensity UV generation in BBO, for example, color center or other defects may be induced and accumulated in an early stage before a sudden damage happen.
In U.S. Pat. No. 5,825,562 to Lai et al, a continuous motion is used to prolong usage of optical elements under the irradiation of intensive laser beam. In an embodiment of UV generation in BBO crystal, Lai et al describe to scan continuously the crystal in a circular area and the usage life of the crystal has been increased significantly. The continuous motion enables a much larger interaction area of the laser beam on the crystal surface and increases the time interval for laser induced thermal or mechanical impacts to relax at any specific spot on the crystal surface. The continuous motion may also allow the laser induced color center and other defects to relax before these defects accumulate and become permanent.
One problem stemming from a continuous motion is a possible fluctuation in the generated UV beam, due to imperfection on the surface of or inside the crystal. Experiment has found it not realistic to expect a crystal having no initial defect and no dust particle over a relatively large area. Also, micro-crack or other micro-defect may grow under irradiation of intensive laser beam or generated UV laser beam. Besides, experiment shows that motion-induced fluctuation in deep UV generation is small but can be troublesome to delicate applications like photo-refractive surgery. This motion-induced fluctuation may stem from residual variation in mechanical alignment and non-uniformity of material or coating of the optical element.
On the other hand, a prior art practice displaces the UV crystal step by step while at each step the crystal stays for a predetermined exposure time or until a predetermined degradation. The crystal displaces to each step location only once and the step location can not be reused after the location has been exposed. This practice works well if the crystal degradation time is relatively long, say several hours or days. This practice becomes not practical if the crystal degradation time is short, say several minutes or tens of minutes.